1. Field of Invention
The present invention relates generally to the field of command, control, communications, computer, intelligence surveillance, and reconnaissance (C4ISR), navigation and timekeeping hardware and software systems and components, and in particular using Global Positioning System (GPS) spread-spectrum communications.
2. Description of Related Art
TTNT (Tactical Targeting Networking Technology) is an advanced tactical data link currently under development by the assignee of the present invention, Rockwell Collins Government Systems and the Advanced Technology Center. Modes supporting Low Probability of Detection (LPD) are a highly desirable addition to existing TTNT functionality. The primary challenge for an LPD receiver is to operate at extremely low signal-to-noise ratio (SNR), often well below negative 20 dB. Because an LPD system must operate at extremely low SNR, the known sequence of chips used for signal acquisition must be very long (on the order of 1 million chips) in order to produce reasonable probabilities of detection and false alarm. While GPS spoofer threats as defined herein are typically irrelevant to TTNT performance, extra copies of TTNT signals would affect TTNT, and, in addition, signal acquisition and signal tracking is affected by GPS spoofer threats. In addition, LPD performance is affected by jamming techniques from GPS threats. Rockwell Collins has successfully demonstrated beam-forming antennas and adaptive digital signal processing techniques to mitigate the impact of traditional jammer threats. Emerging GPS threats, such as repeaters and spoofers, attempt to deceive the user with false signals comprising copies of the original true signal, rather than denying the user access to the GPS signals, a threat that is termed “spoofing”. The same effect can occur naturally from ghost signals from multipath echoes of the original (real) signal.
The Global Positioning System (GPS) is a US Department of Defense satellite based navigation system presently made of 24 satellites in orbit; the first satellite was launched in 1978. Each satellite lasts approximately 10 years, weighs about 2000 pounds, and transmits 50 W low power signals, comprising radio signals L1, L2, that are used for navigation.
Rockwell Collins is developing advanced acquisition techniques that use signal time and frequency consistency to allow the receiver to reject inconsistent copies of the original GPS signal during the acquisition process.
In the present invention, certain terms are used as appreciated by a skilled artisan. Thus “chip” is often defined as “channel bit”. A spread spectrum system, such as used by the present invention, achieves its spectral spreading using one or more techniques such as direct sequence, forward error correction, and orthogonal channel coding. Regardless of the technique used, the bits produced by the spreading are often referred to as “chips”. These chips are modulated and sent over the channel. This distinguishes the bits created by the spreading technique (“chips”) from the information bits going into the spreading technique (“bits”). Note that spread spectrum chips are not required to be binary. “Chip rate” is the rate or frequency at which the chips are transmitted. In a spread spectrum system, the chip rate is much faster than the information bit rate, thus the spectral spreading. “Chip time” is the reciprocal of the chip rate, or the duration in time of a single chip. “Multiple chip times” refers to a period of time that is equal to more than one chip time. A “known sequence” is a sequence of chips (or bits, or symbols) of which an authorized receiver has prior knowledge. The known sequence is typically sent at the beginning of a transmission. The receiver performs a search for the known sequence in order to detect the presence of a desired signal and synchronize its signal processing to it. The process of detecting the presence of a desired signal is often referred to as the signal “acquisition”. Signal acquisition precedes signal demodulation. After signal acquisition comes signal tracking, which can be defined for GPS tracking as the process of continuously estimating the phase of the GPS signal.